Skip to content

Account for impossible bounds making seemingly unsatisfyable dyn-to-dyn casts #145620

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 2 commits into from
Aug 25, 2025
Merged

Account for impossible bounds making seemingly unsatisfyable dyn-to-dyn casts #145620

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
c957c4e
Account for impossible bounds making seemingly unsatisfyable dyn-to-d…
compiler-errors Aug 19, 2025
e0fb6eb
Nits
compiler-errors Aug 23, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
136 changes: 74 additions & 62 deletions compiler/rustc_borrowck/src/type_check/mod.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,7 @@ use rustc_mir_dataflow::points::DenseLocationMap;
use rustc_span::def_id::CRATE_DEF_ID;
use rustc_span::source_map::Spanned;
use rustc_span::{Span, sym};
use rustc_trait_selection::infer::InferCtxtExt;
use rustc_trait_selection::traits::query::type_op::custom::scrape_region_constraints;
use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
use tracing::{debug, instrument, trace};
Expand Down Expand Up @@ -1455,68 +1456,79 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
}
CastKind::PtrToPtr => {
let ty_from = op.ty(self.body, tcx);
let cast_ty_from = CastTy::from_ty(ty_from);
let cast_ty_to = CastTy::from_ty(*ty);
match (cast_ty_from, cast_ty_to) {
(Some(CastTy::Ptr(src)), Some(CastTy::Ptr(dst))) => {
let src_tail = self.struct_tail(src.ty, location);
let dst_tail = self.struct_tail(dst.ty, location);

// This checks (lifetime part of) vtable validity for pointer casts,
// which is irrelevant when there are aren't principal traits on
// both sides (aka only auto traits).
//
// Note that other checks (such as denying `dyn Send` -> `dyn
// Debug`) are in `rustc_hir_typeck`.
if let ty::Dynamic(src_tty, _src_lt, ty::Dyn) = *src_tail.kind()
&& let ty::Dynamic(dst_tty, dst_lt, ty::Dyn) = *dst_tail.kind()
&& src_tty.principal().is_some()
&& dst_tty.principal().is_some()
{
// Remove auto traits.
// Auto trait checks are handled in `rustc_hir_typeck` as FCW.
let src_obj = Ty::new_dynamic(
tcx,
tcx.mk_poly_existential_predicates(
&src_tty.without_auto_traits().collect::<Vec<_>>(),
),
// FIXME: Once we disallow casting `*const dyn Trait + 'short`
// to `*const dyn Trait + 'long`, then this can just be `src_lt`.
dst_lt,
ty::Dyn,
);
let dst_obj = Ty::new_dynamic(
tcx,
tcx.mk_poly_existential_predicates(
&dst_tty.without_auto_traits().collect::<Vec<_>>(),
),
dst_lt,
ty::Dyn,
);

debug!(?src_tty, ?dst_tty, ?src_obj, ?dst_obj);

self.sub_types(
src_obj,
dst_obj,
location.to_locations(),
ConstraintCategory::Cast {
is_implicit_coercion: false,
unsize_to: None,
},
)
.unwrap();
}
}
_ => {
span_mirbug!(
self,
rvalue,
"Invalid PtrToPtr cast {:?} -> {:?}",
ty_from,
ty
)
}
let Some(CastTy::Ptr(src)) = CastTy::from_ty(ty_from) else {
unreachable!();
};
let Some(CastTy::Ptr(dst)) = CastTy::from_ty(*ty) else {
unreachable!();
};

if self.infcx.type_is_sized_modulo_regions(self.infcx.param_env, dst.ty) {
// Wide to thin ptr cast. This may even occur in an env with
// impossible predicates, such as `where dyn Trait: Sized`.
// In this case, we don't want to fall into the case below,
// since the types may not actually be equatable, but it's
// fine to perform this operation in an impossible env.
let trait_ref = ty::TraitRef::new(
tcx,
tcx.require_lang_item(LangItem::Sized, self.last_span),
[dst.ty],
);
self.prove_trait_ref(
trait_ref,
location.to_locations(),
ConstraintCategory::Cast {
is_implicit_coercion: true,
unsize_to: None,
},
);
} else if let ty::Dynamic(src_tty, _src_lt, ty::Dyn) =
*self.struct_tail(src.ty, location).kind()
&& let ty::Dynamic(dst_tty, dst_lt, ty::Dyn) =
*self.struct_tail(dst.ty, location).kind()
&& src_tty.principal().is_some()
&& dst_tty.principal().is_some()
{
// This checks (lifetime part of) vtable validity for pointer casts,
// which is irrelevant when there are aren't principal traits on
// both sides (aka only auto traits).
//
// Note that other checks (such as denying `dyn Send` -> `dyn
// Debug`) are in `rustc_hir_typeck`.

// Remove auto traits.
// Auto trait checks are handled in `rustc_hir_typeck` as FCW.
let src_obj = Ty::new_dynamic(
tcx,
tcx.mk_poly_existential_predicates(
&src_tty.without_auto_traits().collect::<Vec<_>>(),
),
// FIXME: Once we disallow casting `*const dyn Trait + 'short`
// to `*const dyn Trait + 'long`, then this can just be `src_lt`.
dst_lt,
ty::Dyn,
);
let dst_obj = Ty::new_dynamic(
tcx,
tcx.mk_poly_existential_predicates(
&dst_tty.without_auto_traits().collect::<Vec<_>>(),
),
dst_lt,
ty::Dyn,
);

debug!(?src_tty, ?dst_tty, ?src_obj, ?dst_obj);

self.sub_types(
src_obj,
dst_obj,
location.to_locations(),
ConstraintCategory::Cast {
is_implicit_coercion: false,
unsize_to: None,
},
)
.unwrap();
}
}
CastKind::Transmute => {
Expand Down
16 changes: 16 additions & 0 deletions tests/ui/coercion/fake-sized-ptr-cast.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// Make sure borrowck doesn't ICE because it thinks a pointer cast is a metadata-preserving
// wide-to-wide ptr cast when it's actually (falsely) a wide-to-thin ptr cast due to an
// impossible dyn sized bound.

//@ check-pass

trait Trait<T> {}

fn func<'a>(x: *const (dyn Trait<()> + 'a))
where
dyn Trait<u8> + 'a: Sized,
{
let _x: *const dyn Trait<u8> = x as _;
}

fn main() {}
Loading